1. Field of the Invention
The present invention relates to a liquid crystal display which is mainly used in an office automation equipment such as a word processor or a notebook type personal computer, various visual apparatus or a game machine.
2. Description of the Prior Art
A visual apparatus is widely used over a large field such as a watch, a desk calculator, a computer terminal, a word processor or a TV at the present time. A typical display mode used in these uses is a TN (Twisted Nematic) mode in which a liquid crystal molecule in a liquid crystal cell is twisted by almost 90.degree. as initial orientation. In the TN mode a liquid crystal cell is arranged between a pair of polarizers and monochromatic display is provided using an optical property of this cell, more specifically, using optical rotatory power when no voltage is applied and a depolarization characteristic when a voltage is applied. In addition, in the case of coloring, for example red, blue and green color filters are provided in the liquid crystal cell and then multicolor display or full-color display is provided by additive color mixture using the above described optical switching characteristic of the TN mode. The principle therein is used in display of a pocket-sized liquid crystal television using active matrix drive or simple matrix drive.
As a display mode widely used in display for a word processor, there are the TN mode and a STN (Super Twisted Nematic) mode in which a twist angle of the liquid crystal is set at 180.degree. to 270.degree. in the cell structure similar to the TN mode. The feature of this mode is that abrupt molecular orientation deformation with an increase of an applied voltage is reflected in a birefringence change by increasing the twist angle of the liquid crystal 90.degree. or more and optimizing a deflecting plate setting angle, whereby an electro-optical characteristic having sharp threshold value is implemented. Therefore, it is suited to the simple matrix drive. On the other hand, the defect of this mode is that a background of display has color of yellow green or dark blue because of birefringence of the liquid crystal. In order to improve this defect, it is proposed that color compensation is made by piling up a panel for optical compensation or a phase difference plate formed of a high polymer such as polycarbonate on an STN panel for display, whereby white/black display can be provided. Now, this panel structure is on the market as "paper white LCD". In addition, in the case of coloring, multicolor/full-color display can be provided by the same operation principle as in the above TN mode.
When wide viewing angle is required, a GH (guest host) mode is used, in which a dye (dichromatic dye) having different absorbance between a molecule major axis direction and a minor axis direction is added to the liquid crystal. This mode is classified into the Heilmeier type using the polarizer, the White-Taylor type (phase change type) and two-layer type. In any case, as an operation principle, orientation of the dye is controlled by orientation of the liquid crystal molecule by a voltage and an absorbance difference in the direction of dye molecule is used for display. Further, in the case of coloring, as the dye, the dye absorbing a part of wavelength of visible radiation is used or the GH cell using a black dye is combined with a color filter, whereby display can be provided. In addition, detailed operation principles of the TN mode, the STN mode and the GH mode are described in "Liquid Crystal Device Handbook" (1989) pp. 315-346 of No. 142 Committee of the Japan Society for the Promotion of Science.
In the display mode using the polarizer, a utilization factor of light is decreased at least 50% because of the optical characteristic of the polarizer. Therefore, in order to implement "bright display" which is required in reflection type display or projection type display, a display with no polarizer mode is advantageous. In this respect, the White-Taylor type (phase change type) GH mode and the two-layer type GH mode among the above display modes are preferable. When both are compared, it is found that the latter is superior in contrast and brightness as shown in Proc. of the SID, 2514 (1984) pp. 275. However, since the conventional two-layer GH cell uses a normal glass substrate between the liquid crystal layers, parallax is generated in the upper and lower liquid crystal layers depending on a viewing angle, causing display blur. In order to solve this problem, it is thought that a glass thickness between the liquid crystal layers should be extremely decreased. However, it is difficult to handle it and also it is difficult to control the thickness of the cell. Therefore, it can not be applied to highly precise display having a comparatively large area. With regard to this respect the White-Taylor type GH mode is suitable because it is formed of one layer. However, it is inferior in display function as described above. Particularly, in the case of polychromatic display, because it is necessary to use a microcolor filter, brightness is lost and then brilliance of the display is reduced.